The present invention relates to an electronic imaging system and, more particularly, to electronic imaging apparatus, in which pixel signals are read out from a solid-state image sensor having a two-dimensional pixel array and used for recording a still picture or for dynamic image processing.
With recent advancement of fine processing techniques, solid-state image sensors such as CCDs for converting optical image to electric image signal, have a trend for increasing pixel density.
In view of the pixel density, as such high pixel density CCD, an inter-line solid-state image sensor with a pixel density corresponding to, for instance, 1,000,000 pixels, is capable of providing the image at a frame rate of 10 to 15 frames per second when driven by sequential scan driving at a frequency of 20 MHz (megahertz) or below.
The frame rate noted above, however, is not sufficient for the case of using the image data as dynamic image data. Specifically, with such image data an electronic imaging system, which employs the above solid-state imaging element and includes a control system requiring dynamic image data, cannot fast execute such processes as auto focus (AF) control, auto exposure (AF) control and auto white balance (AWB) control.
This will now be described with reference FIG. 24 of the accompanying drawings.
Referring to FIG. 24, which is a time chart illustrating CCD output data and liquid crystal display data obtainable by prior art CCD drive means, labeled 1/60 VD is a vertical sync signal with a cycle time of 1/60 seconds. Under control of this vertical sync signal, output data illustrated as A, B, C, D, E, . . . are obtained from a CCD at a frame rate of 10 frames per second.
In a system with a liquid crystal display, it is necessary to set a frame rate of about 60 frames per second for liquid crystal display driving. Otherwise, satisfactory display cannot be obtained for such reason as the reaction of crystal. To solve this problem, the display frame rate for display is increased by storing each frame of CCD output data in a memory or the like and providing the stored frame data a certain number of times.
Specifically, data A, for instance, outputted from the CCD and stored in the memory, is provided as liquid crystal display data repeatedly, six times, for instance. Since the data A is provided at a rate of 10 frames per second as noted above, the liquid crystal display data is provided at a rate of 60 frames per second.
The above drive means always require provision of a memory or like means. From the standpoint of the cost, however, it is not desirable to use an expensive memory for the sole frame rate control purpose.
Another problem is encountered when using the above CCD output data as control data for processing with high rate dynamic image data, for instance AF (auto focus) data as shown in parentheses as CCD output data in FIG. 24, or as control data for AE or AWB.
With a single frame of CCD output data, AF control, for instance, cannot always be obtained. Heretofore, AF has been controlled by using a plurality of frames. Dynamic image data at the frame rate of only 10 frames per second as noted above, requires long AF time.
Particularly, time-consuming AF, AE or AWB is very disadvantageous for an electronic still camera used as electronic imaging system for taking still pictures, because it is very important with such camera to time shutter chances without missing. In this viewpoint, the time-consuming AF, AE or AWB is very disadvantageous.
Besides, where the obtainable frame rate is only about 10 frames per second as noted above, it is necessary to execute the AF, AE and AWB controls in parallel processes in order for the controls to be made in due time. A memory or the like should be used or the circuit scale should be increased, which is disadvantageous from the standpoint of the cost.
In order to increase the frame rate without use of any memory, it is conceivable to have resort to a thin-down scan read means. In the prior art, thin-down scan reading is made in a manner, which will now be described with reference to FIGS. 25 and 26.
FIG. 25 is a view illustrating the configuration of an example of color filter in a prior art solid-state image sensor.
The line sequential filter configuration shown in FIG. 25 is well known as Bayer array of original colors. In this array, a 1-st line consists of alternate red (R) and green (G) pixels. A 2-nd line consists of alternate green (G) and blue (B) pixels. Subsequent odd lines have the same configuration as the 1-st line, and subsequent even lines have the same configuration as the 2-nd line. This Bayer array of original colors is a color difference line sequential filter array, which is usually used in a commonly called whole pixel read CCD.
FIG. 26 illustrates a purely every other line thin-down scan, which is usually conceivable for increasing the frame rate in a CCD by a thinning-down process.
By such thin-down scan read, however, only odd lines, for instance 1-st, 3-rd, 5-th, etc. lines, are read out. Therefore, with a CCD having the Bayer array filter configuration as shown in FIG. 25, only RG lines (containing red information) shown as (CR) in FIG. 26, are read out. In other words, blue (B) signal (containing blue information) present in even lines (as shown as (CB) in FIG. 26) cannot be obtained.
As shown above, the purely every other line thin-down scan read for increasing the frame rate cannot permit all of G, R and B signals to be obtained at all times for increasing the frame rate. Therefore, the CCD output data cannot be used as color data for the liquid crystal display purpose, that is, it cannot be suitably used for an electronic still camera which has a liquid crystal display.
As described above, for driving a high pixel density inter-line solid-state image sensor at a frame rate suitable for dynamic image signal processing, a high drive frequency is necessary, and the above frame rate is insufficient in sequential scan driving at a frequency of 20 MHz or below.
The thin-down scan is conceived to solve this problem. As described before, however, in an whole pixel CCD image sensor which usually uses a color filter having a Bayer array or like color difference line sequential array, the thin-down scan gives rise to missing of color signal, making it impossible to obtain necessary color signal.